As a tropical downpour loomed, about 400 people stayed put to spell out "SOS" next to a huge banner that read "Reef in Danger" on the city’s Esplanade on March 11.
The rally marked the visit to the city of a United Nations Educational, Scientific and Cultural Organization (UNESCO) commission, which looked at the possible impacts of the dramatic rise in shipping through the Great Barrier Reef expected over the next decade.
UNESCO has responsibility for the World Heritage listing for the reef.
The 2010 grounding of the coal ship Shen Nang 1 on the reef and revelations of ecological damage from dredging in Gladstone harbour have highlighted the potential problems.
The main cargo the ships will carry is coal and other mining products. The federal and state governments back the expansion of mining in central and northern Queensland.